BN Nanosheet/Polymer Films with Highly Anisotropic Thermal Conductivity for Thermal Management Applications Yuanpeng Wu Deakin University

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Australian Institute for Innovative Materials - Papers Australian Institute for Innovative Materials

2017 BN Nanosheet/Polymer Films with Highly Anisotropic Thermal Conductivity for Thermal Management Applications Yuanpeng Wu Deakin University

Ye Xue Rowan University

Si Qin Deakin University

Dan Liu Deakin University

Xuebin Wang Nanjing University

See next page for additional authors

Publication Details Wu, Y., Xue, Y., Qin, S., Liu, D., Wang, X., Hu, X., Li, J., Wang, X., Bando, Y., Golberg, D., Chen, Y., Gogotsi, Y. & Lei, W. (2017). BN Nanosheet/Polymer Films with Highly Anisotropic Thermal Conductivity for Thermal Management Applications. ACS Applied Materials and Interfaces, 9 (49), 43163-43170.

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Abstract The development of advanced thermal transport materials is a global challenge. Two-dimensional nanomaterials have been demonstrated as promising candidates for thermal management applications. Here, we report a boron nitride (BN) nanosheet/polymer composite film with excellent flexibility and toughness prepared by vacuum-assisted filtration. The mechanical performance of the composite film is highly flexible and robust. It is noteworthy that the film exhibits highly anisotropic properties, with superior in-plane thermal conductivity of around 200 W m -1 K -1 and extremely low through-plane thermal conductivity of 1.0 W m -1 K -1 , making this material an excellent candidate for thermal management in electronics. Importantly, the composite film shows fire-resistant properties. The newly developed unconventional flexible, tough, and refractory BN films are also promising for heat dissipation in a variety of applications.

Disciplines Engineering | Physical Sciences and Mathematics

Authors Yuanpeng Wu, Ye Xue, Si Qin, Dan Liu, Xuebin Wang, Xiao Hu, Jingliang Li, Xungai Wang, Yoshio Bando, Dmitri Golberg, Ying I. Chen, Yury Gogotsi, and Weiwei Lei

This journal article is available at Research Online: http://ro.uow.edu.au/aiimpapers/2895 Page 1 of 26 ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 BN nanosheet/polymer films with highly 8 9 10 11 anisotropic thermal conductivity for thermal 12 13 14 15 16 management application 17 18 19 20 Yuanpeng Wu1,2, Ye Xue3, Si Qin1, Dan Liu1, Xuebin Wang4,6, Xiao Hu3, Jingliang Li1, Xungai 21 22 Wang1, Yoshio Bando4, Dmitri Golberg4,7, Ying Chen1, Yury Gogotsi5, Weiwei Lei1* 23 24 1 25 Institute for Frontier Materials, Deakin University, Waurn Ponds Campus, Locked Bag 20000, 26 27 Victoria 3220, Australia 28 2 School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 29 30 610500, China 31 3 32 Department of Physics and Astronomy and Department of Biomedical Engineering, Rowan 33 University, 201 Mullica Hill Road, Glassboro, New Jersey 08028, United States 34 35 4 International Center for Materials Nanoarchitectonics (WPIMANA), National Institute for 36 37 Materials Science (NIMS), Namiki 11, Tsukuba, Ibaraki 3050044, Japan 38 5 39 A. J. Drexel Nanomaterials Institute, and Materials Science and Engineering Department, 40 Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, United States 41 42 6 College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China 43 7 44 School of Chemistry, Physics and Mechanical Engineering Science and Engineering Faculty, 45 46 Queensland University of Technology, Brisbane, QLD 4001, Australia 47 48 49 50 51 KEYWORDS: BN nanosheet, poly (diallyl dimethyl ammonium chloride), composite film, 52 53 thermal conductivity 54 55 56 57 58 59 60 ACS Paragon Plus Environment 1 ACS Applied Materials & Interfaces Page 2 of 26

1 2 3 ABSTRACT: Development of advanced thermal transport materials is a global challenge. Two 4 5 6 dimensional nanomaterials have been demonstrated as promising candidates for the thermal 7 8 management applications. Here, we report a boron nitride (BN) nanosheet/polymer composite 9 10 film with excellent flexibility and toughness prepared by vacuumassisted filtration. The 11 12 mechanical performance of the composite film is highly flexible and robust. It is noteworthy that 13 14 15 the film exhibits highly anisotropic properties, with superior inplane thermal conductivity 16 17 around 200 W m1 K1 and extremely low throughplane thermal conductivity (1.0 W m1 K1), 18 19 making this material an excellent candidate for thermal management in electronics. Importantly, 20 21 22 the composite film shows fireresistance properties. The newly developed unconventional 23 24 flexible, tough, and refractory BN films are also promising for heat dissipation in a variety of 25 26 applications. 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 2 Page 3 of 26 ACS Applied Materials & Interfaces

1 2 3 4 5 6 INTRODUCTION 7 8 9 The development of nanomaterials with anisotropic thermal transport properties has attracted 10 11 increasing attention for thermal management applications, such as coatings in electronics and 12 13 heat sinks.1 An ideal candidate should be electrically insulating and have thermal conductivity 14 15 1 16 inplane significantly greater than throughplane. Twodimensional (2D) nanomaterials such as 17 18 graphene and black phosphorus, both of which possess highly anisotropic thermal conductivity, 19 20 have recently provided a new platform for addressing heat dissipation in devices.2,3 However, it 21 22 23 is difficult to employ either in thermal management or build components owing to their electrical 24 25 conductivity and fast oxidation of phosphorus. BN nanosheets offer a high thermal conductivity, 26 27 large and direct band gap, resistance to oxidation, high elastic modulus, and a low friction 28 29 410 30 coefficient. BN nanosheets have been widely explored for use in functional nanocomposite 31 1115 32 materials, ultravioletlight emitters, hydrogen storage, and sorbents. In contrast with other 33 34 conductive and semiconducting 2D materials, such as graphene, transition metal 35 36 dichalcogenides, and MXenes, BN nanosheets are electrically insulating, which suggests utility 37 38 39 in thermal management applications in electronics. 40 41 42 Much effort has recently been devoted to the preparation of highly flexible, strong, and tough 43 44 graphene, clay, and MXene films for applications in gas separation, molecular and ionselective 45 46 1620 47 devices, and energy storage. However, pure BN nanosheet films do not offer strength and 48 49 flexibility required in reallife application. Creating nanocomposite with polymers is an effective 50 51 approach to improve the mechanical properties of nanomaterials.21,22 There have been many 52 53 5,2325 54 studies on BNpolymer nanocomposites with high polymer contents. However, the total 55 56 strength, elongation and thermal conductivities of such composites are largely controlled by the 57 58 59 60 ACS Paragon Plus Environment 3 ACS Applied Materials & Interfaces Page 4 of 26

1 2 3 polymer, staying far from the inherent properties of BN. To date, some studies have 4 5 6 demonstrated that 2D materials such as MXene and graphenebased nanocomposites with low 7 16,26,27 8 contents of polymer exhibit enhanced mechanical strength and electrical conductivity. 9 10 Therefore, to fully take advantage of the extraordinary properties of these 2D materials, the 11 12 content of polymer in the composite should be limited. BN nanosheetbased composites with 10 13 14 15 wt% polyvinyl alcohol (PVA) have recently been fabricated, exhibiting an excellent tensile 16 17 strength (125.2 MPa).28 However, the reported composites show a low thermal conductivity of 18 19 less than 6.9 W m1 K1 owing to the surfactant involved and the thickness of the nanosheet. 20 21 22 Therefore, developing a BN/polymer composite film with the flexibility of a polymer, superior 23 24 thermal conductivity of BN and the high mechanical strength remains a challenge. Additionally, 25 26 the fire hazard of polymeric materials restricts their application in thermal management fields, 27 28 29 which necessitates the addition of fire resistance. Recent works have shown that graphene oxide 30 31 and clay can be considered fire resistance while simultaneously improving the mechanical and 32 33 thermal properties of the composite.29,30 This justifies the use BN nanosheets, which are 34 35 chemically inert and oxidation resistant,31 in the construction of multifunctional fireresistance 36 37 38 nanocomposites. 39 40 41 Here, we report a freestanding BN nanosheet/polymer composite film with a unique 42 43 combination of properties including flexibility and toughness, anisotropic thermal conductivity, 44 45 46 and fire resistance, which is produced by the vacuumassisted filtration (VAF) of colloidal 47 48 suspensions of BN nanosheets and polymers (Figure 1a). In this work, poly (diallyl dimethyl 49 50 ammonium chloride) (PDDA) was chosen, which is a cationic polymer that combines with 51 52 26 53 negatively charged the BN nanosheet through electrostatic interactions. Owing to the merits, 54 55 56 57 58 59 60 ACS Paragon Plus Environment 4 Page 5 of 26 ACS Applied Materials & Interfaces

1 2 3 the BN film exhibit highly potential applications in highperformance flexible electrically 4 5 6 insulating substrates, superior thermal conductivities and fireresistance coatings. 7 8 9 EXPERIMENTAL 10 11 12 Preparation of boron nitride (BN) nanosheets. BN nanosheets were prepared using a 13 14 32 15 previously reported method. Typically, hBN powder with the size 2–10 m (Momentive 16 17 Performance Materials, Inc.) and urea (SigmaAldrich) were mixed in a steel milling container 18 19 using a planetary ball mill (Pulverisette 7, Fritsch). The mixture with a weight ratio of 1:20 (h 20 21 22 BN: urea) was protected by nitrogen and milled at a rotation speed of 500 r.p.m. for 20 h. The 23 24 obtained powders were dissolved in water and dialyzed for 1 week in deionized water to remove 25 26 the urea, yielding stable aqueous dispersions of BN nanosheets with the size 200300 nm. 27 28 29 Preparation of BN/PDDA films. BN/PDDA films were fabricated by vacuumassisted filtration 30 31 1 32 (VAF). To avoid the agglomeration, the BN dispersion (2 mg mL ) was diluted more than 10 33 34 times, then slowly mixed with PDDA solution to form a BN and PDDA dispersion. To form 35 36 composite films, the mixture was filtered through a glass microfiltration apparatus (Sigma) of ca. 37 38 39 36 mm as 10, 20, 30, 40, 50, and 60 wt%, and the resulting films were denoted as BN/PDDA 10, 40 41 20, 30, 40, 50, and 60 wt%, respectively. Pure BN films were produced using a similar approach 42 43 and were denoted in diameter with a polyethylene membrane under vacuum assistance. These 44 45 46 films can be readily peeled off from the polyethylene membrane and retain their freestanding 47 32 48 state. The mass fractions of PDDA in the BN and PDDA mixtures were set as BN. 49 50 51 Material characterization. XRD measurements were performed on a PAN alytical X’Pert PRO 52 53 α 54 diffractometer operating with Cu K radiation. SEM analysis was performed on a Zeiss Supra 55 56 55 VP SEM instrument. TEM and HRTEM imaging were performed on a JEOL 2100F 57 58 59 60 ACS Paragon Plus Environment 5 ACS Applied Materials & Interfaces Page 6 of 26

1 2 3 microscope operating at 200 kV. The FTIR and optical transmittance spectra were recorded 4 5 6 using a Nicolet 7199 FTIR spectrometer and Cary 3 spectrophotometer, respectively. The 7 8 thermal behavior was analyzed using TGA on a TA Instruments Q50 TGA thermal analyzer at a 9 10 heating rate of 10 °C min1 from room temperature to 800 °C under 60 sccm compressed air 11 12 flow. 13 14 15 16 Mechanical testing. The BN/PDDA films and BN films were cut into stripes measuring 25 mm 17 18 × 5 mm in size and glued onto supporting paper frames. These stripes were fixed in the grips of a 19 20 universal testing machine (Instron 2360), and then the paper frames were cut and the tensile tests 21 22 23 were performed using a 50N load cell with a crosshead rate of 5 mm/min. These mechanical 24 25 tests were conducted at room temperature under a humidity of approximately 45%. The Young’s 26 27 modulus, tensile strength, and strain to failure were calculated as the averages of the results of 28 29 30 five parallel experiments. 31 32 33 The hollow cylinders of BN/PDDA 30% films were produced by rolling the corresponding 34 35 films (34 mm × 10 mm) around a glass rod (6 mm diameter) and gluing the edges of the strips 36 37 with a small amount of PDDA solution (0.1 wt%). These hollow cylinder specimens were dried 38 39 40 in a vacuum oven and weighed, and then they were fixed on a glass slide by PDDA solution (1 41 42 wt%) for evaluating the weight loading. 43 44 45 Thermal conductivity. The inplane thermal conductivity of the BN/PDDA films was measured 46 47 48 by a Physical Property Measurement System (PPMS, Quantum Design, USA) using the steady 49 50 state method (Figure S1). A rectangular (4 mm ×18 mm) film sample with a radiation shield was 51 52 placed in a vacuum chamber (9.4×105 Torr) to minimize the radiation, convection, and 53 54 55 conduction heat loss. The radiation heat loss is also calculated by the measurement system. A 56 57 58 59 60 ACS Paragon Plus Environment 6 Page 7 of 26 ACS Applied Materials & Interfaces

1 2 3 heater shoe was connected to one end of the sample, and the other end was fixed to a cold sink. 4 5 6 The hot thermometer shoe and cold thermometer shoe were both connected to the measured 7 8 sample at a distance L (as shown in Figure S1). At steady state, the thermal conductivity (K) of 9 10 the sample is determined by equation 11 12 13 K=(Q×L)/(A×T), 14 15 where Q is the net heat flowing through a known cross section A of the measured sample, ΔT is 16 17 the temperature difference between the hot thermometer and cold thermometer, and L is the 18 19 20 distance between the two thermometer shoes. 21 22 23 The thermal conductivity (throughplane direction) of the BN/PDDA films was analyzed by 24 25 the device of aiPhase Mobile 1u using a temperaturewaveanalysis method according to ISO 26 27 28 220073 (Supplementary Methods). The samples were sandwiched between the heater and sensor 29 30 plates. By scanning the frequency of heat source, the delay in the phase of the temperature wave 31 32 was recorded within the range from 180o to 230o based on an empirical criterion. The linear 33 34 35 fitting of phase lag θ versus square root of frequency, sqrt(f), was conducted to calculate thermal 36 37 diffusivity a following equation 38 39 40 θ= d×sqrt(f/a), 41 42 43 44 where d is the thickness of samples. The thermal conductivity was further calculated based on 45 46 the thermal diffusivity as well as theoretical density and specific heat capacity of composites. 47 48 The experimental errors were estimated by means of the squarerootsum error propagation 49 50 51 approach, including the following error sources: the measurement of thickness of samples and 52 53 the physical property measurement system. 54 55 56 57 58 59 60 ACS Paragon Plus Environment 7 ACS Applied Materials & Interfaces Page 8 of 26

1 2 3 Fire-resistance testing. A parallel flame test was performed on pure PDDA, BN/PDDA 10% 4 5 6 and 30% films with dimensions of 20 mm in length and 5 mm in width. The fire from a gas 7 8 burner was applied on the tested film for 60 s and then removed. 9 10 11 RESULTS AND DISCUSSION 12 13 14 In this work, watersoluble functionalized boron nitride nanosheet having negative surface 15 16 17 charge and sizes of around 200300 nm were prepared according to our previously reported 18 19 method (Figure S2).32 The negativelycharged BN nanosheets attracts positivelycharged PDDA 20 21 through electrostatic interactions, producing a uniform hybrid structure. The BN/PDDA 22 23 24 composite films can be easily peeled off from the filter membrane after filtration to form free 25 26 standing paperlike laminated films. The thickness and size of the BN/PDDA composite films 27 28 can be easily adjusted by the feed amounts of the colloidal suspensions of the BN nanosheets and 29 30 31 PDDA and the size of the filtration system, respectively. 32 33 34 The films are nearly transparent (Figure 1a and Figures S3), smooth, and flexible and can be 35 36 readily processed into desired shapes (Figure 1b). The transparency of the films was further 37 38 demonstrated using UVVis spectroscopy (Figure S4). These composite films can be easily 39 40 41 reshaped and made more flexible by increasing the content of PDDA. This is in stark comparison 42 43 to the pristine BN film, which is weak and cannot be folded into desired shapes. The flexibility 44 45 of the BN/PDDA film mainly comes from the strong interactions of the BN nanosheets with the 46 47 48 PDDA through electrostatic interactions (Figure 1c). 49 50 51 SEM and TEM were used to characterize the morphology of the films. The pure PDDA film 52 53 is homogenous, while the BN/PDDA films are composed of parallelstacked layers (Figure 1df). 54 55 56 The BN nanosheets and PDDA chains in the composite films are alternating, indicating that they 57 58 59 60 ACS Paragon Plus Environment 8 Page 9 of 26 ACS Applied Materials & Interfaces

1 2 3 disperse in the aqueous solution homogenously and form compact films when filtered, as shown 4 5 6 in the TEM image (Figure 1g and Figure S5). In addition, the spacing between the layers 7 8 decreases with decreasing amounts of PDDA to 10 wt% (Figure S6) indicating the intercalation 9 10 effects of PDDA into BN layers. 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 Figure 1. (a) Photo of a freestanding BN/PDDA 30% film. (b) Photo of a scrolled BN/PDDA 46 47 30% film. (c) Schematic illustration of composite film preparation by VAF. (d) Crosssectional 48 49 50 SEM images of PDDA and (e, f) BN/PDDA 30% films. (g) Crosssectional TEM image of 51 52 BN/PDDA 30% film. (h) XRD patterns of BN/PDDA films. 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 9 ACS Applied Materials & Interfaces Page 10 of 26

1 2 3 The structures of the BN/PDDA films were further analyzed by Xray diffraction (XRD). 4 5 6 One characteristic diffraction peak can be observed at 26.2°, which arises from the (002) plane of 7 8 BN (Figure 1h). With the increasing incorporation of PDDA, the (002) peak shifts to a higher 9 10 diffraction angle and exhibits a remarkably reduced intensity, indicating that the spacing between 11 12 the layers of BN slightly decreases with the increasing amount of PDDA owing to the stacking 13 14 15 force from the PDDA, but that fewer stacked BN layers are present. PDDA layers separate most 16 17 flakes. This is consistent with the TEM results and further demonstrates that the PDDA can enter 18 19 into the space between BN nanosheets.26,33 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 Figure 2. FTIR spectra of PDDA, BN/PDDA10%, 20%, 30%, 40%, 50%, 60% and pure BN 43 44 films. 45 46 47 The Fouriertransform infrared (FTIR) spectrum of the BN/PDDA films (Figure 2) show the 48 49 50 two strong characteristic absorption bands of BN nanosheets of the inplane BN stretching 51 1 52 vibrations at approximately 1360 cm and the outofplane BNB bending vibrations at 53 54 approximately 755 cm1.34 Moreover, a broad peak from 3000 ~ 3600 cm1 can been seen in the 55 56 57 58 59 60 ACS Paragon Plus Environment 10 Page 11 of 26 ACS Applied Materials & Interfaces

1 2 3 inset due to the NH (approximately 3250 cm1) and OH (approximately 3410 cm1) stretching 4 5 38,39 6 vibrations from the fabricating process . The pure PDDA film shows only the characteristic 7 1 1 35 8 CH stretching vibrations at 2973 cm , CN stretching vibrations at approximately 1124 cm , 9 10 and bands from 3150 to 3550 cm1 that are due to the OH stretching vibrations of water 11 12 absorbed in the PDDA chains.36 13 14 15 16 The NH2 and OH groups can also be confirmed by an Xray photoelectron spectroscopy 17 18 (XPS). As shown in Figure S7a, the peak at 190.4 eV corresponds to BN bonds in BN, and the 19 20 shoulder (191.2 eV) should be attributed to the B atoms in B–O bonds formed due to the –OH 21 22 37 23 groups attached to defects along the edges in BN. In Figure S7b, the main peak with a binding 24 25 energy of 398 eV in the N 1s spectrum corresponds to N–B bonds in BN, a shoulder can be 26 27 deconvoluted at 399 eV corresponding to N–H bonds which further confirmed the presence of – 28 29 32 30 NH2 group. 31 32 33 Furthermore, the BN before and after exfoliation exhibit high negative potential value, 44 34 35 and 33 mV, respectively.32 The BN nanosheets exhibits a somewhat decreased absolute value of 36 37 38 zeta potential (33 mV) after functionalization. The reduced negative zeta potential after 39 40 functionalization is due to the partial neutralization effect between positively charged NH3+ 41 42 groups and negatively charged oxygen containing groups formed on BN surface upon contact 43 44 32 45 with water. 46 47 48 Unlike other 2D films, including MXene, graphene oxide and carbon nanotubebased 49 50 films,26,37,38 which exhibit good mechanical properties, pure BN nanosheet films are usually 51 52 53 brittle and difficult to handle due to a small lateral size of BN flakes. The Young’s modulus and 54 55 tensile strength of the BN films are 28.3 ± 10.6 and 7 ± 0.4 MPa, respectively (Figure 3a and 56 57 58 59 60 ACS Paragon Plus Environment 11 ACS Applied Materials & Interfaces Page 12 of 26

1 2 3 Supplementary Table S1). These values are rather low compared with those of the 2D films 4 5 6 mentioned above and far below that of singlecrystal hBN (~36.5 GPa) due to weak bonding 7 39 8 between the multilayer BN sheets in the films. However, upon introducing PDDA, the 9 10 Young’s modulus and tensile strength are greatly improved. As shown in Figure 3a, Figure S8 11 12 and Table S1, upon introducing 10 wt% PDDA into the BN nanosheets films, the Young’s 13 14 15 modulus and tensile strength are improved by approximately 10 times, respectively. The strain to 16 17 failure is also enhanced by the addition of more PDDA (Figure S9). The mechanical properties 18 19 of other BN/PDDA films with different polymer contents are shown in Figure 3a,b, indicating 20 21 22 that the mechanical properties of the composite can be tailored by changing the ratio of BN 23 24 nanosheets to PDDA. The dramatic enhancements in the strength, modulus, and strain to failure 25 26 of these composite films originated from three main reasons: (i) a good dispersion of BN 27 28 32 29 nanosheets in the polymer matrix due to the NH2 and OH groups on their surface; (ii) these 30 31 functional groups could increase the interfacial interaction between BN nanosheets and the 32 33 PDDA through the electrostatic interactions in BN/PDDA. As we reported before, the zeta 34 35 potential of BN is −33 mV.32 Therefore, the BN nanosheets with negative surface charge attract 36 37 40 38 positively charged PDDA, which is similar with the previous report (Figure 1c). When external 39 40 stress is applied to a BN/PDDA composite film, it can easily be transferred from a nanosheet to a 41 42 polymer and from the PDDA to another nanosheet due to the strong interfacial interactions;41 (iii) 43 44 45 intrinsic properties of BN are largely maintained in the films produced by vacuumassisted 46 47 filtration and lead to the inherent damage resistance of the composite films. In summary, 48 49 BN/PDDA films can offer superior mechanical properties and find many applications. 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 12 Page 13 of 26 ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Figure 3. (a) Stressstrain curves of BN, BN/PDDA, and PDDA films. (b) A BN/PDDA 30% 29 30 film supporting ∼17,000 times its own weight. (c) Inplane and throughplane thermal 31 32 conductivities of BN/PDDA films with different contents of PDDA. (d) Thermal conductivity 33 34 35 enhancement of the BN/PDDA composites reported in previous works. 36 37 38 To observe the compressive strengths of the BN/PDDA films, we used hollow cylinders that 39 40 were made by rolling the BN/PDDA films and pasting the overlapping edges with PDDA.26 As 41 42 43 shown in Figure 3b, a hollow cylinder of BN/PDDA 30% (6 mm in diameter and 10 mm high), 44 45 made from a film of 34 mm in length and 10 mm in width can readily support approximately 46 47 17,000 times its own weight without suffering visible deformation, suggesting that their 48 49 compressive strength is very high. The robust compressive strength and toughness of the 50 51 52 composite films further demonstrate the strong interfacial interactions between BN nanosheets 53 54 and polymers through the electrostatic interactions. The high compressive strength and toughness, 55 56 57 58 59 60 ACS Paragon Plus Environment 13 ACS Applied Materials & Interfaces Page 14 of 26

1 2 3 together with the exceptional flexibility, suggest that these BN nanosheetbased composite films 4 5 6 may find applications as structural or functional materials in thermally conductive actuators, 7 16,42 8 flexible energy storage devices, and temperature sensors. 9 10 11 The BN/PDDA films not only exhibit the enhanced mechanical properties of the BN but also 12 13 show highly anisotropic thermal conductivity. The thermal conductivities of BN/PDDA films 14 15 16 with different PDDA weight percentages are presented in Figure 3c. It can be seen that the in 17 18 plane thermal conductivity increases with the decreasing weight fraction of PDDA. The inplane 19 20 thermal conductivity can be up to 212.8 W m1 K1 for BN/PDDA 10% films. Even for 21 22 1 1 23 BN/PDDA 60% films, the inplane thermal conductivity values can reach 105.2 W m K , 24 25 respectively. In order to demonstrate the superiority thermal conductivity of the BN/PDDA film, 26 27 Figure 3d summarizes previously reported thermal conductivity of BN/polymer composites with 28 29 1,6,9,23,24,43–50 30 different BN weight contents in the polymer matrix. One can see that the BN/PDDA 31 32 film exhibits the highest thermal conductivity enhancement with the similar BN weight content 33 34 among the reported BN/polymer composites. Moreover, BN/PDDA 10% is superior to 35 36 conventional airspace materials such as aluminum alloy51 and comparable to aluminum metal. It 37 38 39 is notable that the BN/PDDA films exhibit a large anisotropy of thermal conductivity with an 40 41 extremely low throughplane thermal conductivity (1.0 W m1 K1). The anisotropy of the 42 43 thermal conductivity is due to the layered structure consisting of the BN nanosheets aligned in 44 45 46 the polymers due to selfassembly or BN nanosheets bonded by very thin layers of polymer at 47 48 the high BN contents. The heat transfer strongly depends on the orientation of 2D 49 50 nanomaterials.52 The BN nanosheets were evenly distributed in the crosssection and aligned 51 52 53 parallel to the surface of the films due to the good dispersion of BN nanosheets in the PDDA 54 55 matrix. Therefore, there is a large contact area between the adjacent BN nanosheets (Figure 1eg, 56 57 58 59 60 ACS Paragon Plus Environment 14 Page 15 of 26 ACS Applied Materials & Interfaces

1 2 3 Figure S10) in BNrich films and the heat is preferably transferred along the BN nanosheets 4 5 6 direction. With more BN nanosheets aligned inplane, the interfacial thermal resistance decreases 7 8 and the number of conductive paths for inplane heat transfer increases further. Consequently, 9 10 the BN/PDDA films with a low polymer content exhibit a relatively high thermal conductivity 11 12 inplane. On the contrary, polymers present between the BN nanosheets in throughplane 13 14 53 15 direction lead to slow heat transfer and large anisotropy. Thermally insulating polymer impedes 16 17 the heat transfer in the throughplane direction. Therefore, the BN/PDDA films show a lower 18 19 thermal conductivity along the throughplane direction. 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 Figure 4. (a) Photographs of pure PDDA and (b) BN/PDDA 10% films on a hot flame for 39 40 41 different times. 42 43 44 Many of the commonly used flame resistance are organic materials, which often show poor 45 46 tolerance to oxidants with negative environmental and health effects.54 However, recent work has 47 48 49 shown that the fireresistance of materials can be improved by the addition of 2D materials such 50 51 as clays and graphene oxides. In addition, it is known that BN materials show high resistance to 52 53 oxidation.15 Nanostructured BN is therefore an ideal candidate in the construction of 54 55 multifunctional fireresistance polymer nanocomposites. Here, BN/PDDA films with outstanding 56 57 58 59 60 ACS Paragon Plus Environment 15 ACS Applied Materials & Interfaces Page 16 of 26

1 2 3 electrical insulation, high thermal conductivity, good chemical stability, and oxidation resistance 4 5 6 are promising candidates for the development of new types of fireresistance materials. Parallel 7 8 burning tests (Figure 4, and Figure S11) show that BN/PDDA 1030% films display excellent 9 10 fireresistance, where the films can nearly maintain their initial shape after burning for more than 11 12 60 s. The results indicate that the flameresistance BN nanosheets disperses in polymer matrix 13 14 15 homogenously which enables the preservation of the flameresistance functions of films. 16 17 However, the pure PDDA films ignited on the flame and were burned out within 3s (Figure 4). 18 19 Thermogravimetric analysis (TGA) in air revealed that the oxidation resistance of the BN/PDDA 20 21 22 films increases with the BN content (Figure S11b). The superior flameresistance property of the 23 24 BN/PDDA films may significantly improve safety when they are used as heat shields, flexible 25 26 fireresistant coatings, and electrically insulating substrates. To demonstrate the potential 27 28 29 application of the BN/PDDA films in cooling electronic device, the BN/PDDA 30% film is used 30 31 as a substrate with a size of 40 × 40 mm to fix one lightemittingdiode (LED) chip on the central 32 33 of sample (Figure 5a,b). Silver paste was used for adhesion between the LED chip and samples. 34 35 Polyimides (PI) abstract was selected as a comparison sample, because it has been used widely in 36 37 38 printed circuit boards due to the lightweight and high mechanical toughness. When the PI was as 39 40 the substrate, after steadystate (10 min) of the heat flow caused by the LED chip, a hot spot 41 42 appeared at the center (75°C), with a high temperature gradient from the center to the edge 43 44 −1 −1 45 (Figure 5c) due to the poor thermal conductivity of the PIs (0.3 W m K ). However, the 46 47 BN/PDDA 30% film exhibits a dramatically reduced center spot temperature (52°C) with a more 48 49 uniform temperature distribution under similar conditions (Figure 5d). The BN/PDDA film 50 51 52 exhibits highly anisotropic properties as confirmed by the thermal conductivity measurement 53 54 (Figure 5c). It indicates that the heat is not being conducted well in the throughplane direction, 55 56 57 58 59 60 ACS Paragon Plus Environment 16 Page 17 of 26 ACS Applied Materials & Interfaces

1 2 3 while the heat is being spread inplane direction efficiently. Consequently, the concentrated hot 4 5 6 spot is not clearly present on the BN/PDDA film in Figure 5b, which will provide higher stability 7 8 and reliability of LED. A cooling material should have not only a high thermal conductivity, but 9 10 also an excellent flexibility. In our work, the LED worked well when the device was bended or 11 12 twisted (Figure 5e,f), confirming excellent flexibility of the BN/PDDA film. 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 Figure 5. Application of the BN/PDDA 30% film in flexible substrates. Photos of the PI (a) and 47 48 BN/PDDA 30% film substrates (b). The corresponding thermal images of the PI (c) and 49 50 BN/PDDA 30% film (d). (e, f) Photos of working electronic devices by using a BN/PDDA 30% 51 52 53 film substrate, exhibiting its excellent flexibility. 54 55 56 CONCLUSIONS 57 58 59 60 ACS Paragon Plus Environment 17 ACS Applied Materials & Interfaces Page 18 of 26

1 2 3 In summary, nearly transparent, flexible and tough BN/PDDA films have been manufactured 4 5 6 by simple and scalable vacuum assisted filtration. The Young’s modulus, tensile strength, and 7 8 strain to failure of the BN films improved dramatically after incorporation of PDDA. The 9 10 composite films exhibit highly anisotropic thermal conductivity reaching ~200 W m1 K1 in 11 12 plane and being ~1.0 W m1 K1 in the outofplane direction. Those properties combined with 13 14 15 high chemical and temperature stability make them useful for various applications, particularly 16 17 in thermal management (heat spreading), and flexible and fireresistant coatings. 18 19 20 ASSOCIATED CONTENT 21 22 23 24 Supporting Information. 25 26 27 Illustration of the thermal conductivity setup, additional photos, optical transmittance, FTIR, 28 29 mechanical properties, TGA of BN/PDDA films. This material is available free of charge via the 30 31 32 Internet at http://pubs.acs.org. 33 34 35 AUTHOR INFORMATION 36 37 38 Corresponding Author 39 40 * Corresponding author Email: [email protected] 41 42 43 44 Author Contributions 45 46 The manuscript was written through contributions of all authors. All authors have given approval 47 48 to the final version of the manuscript. 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 18 Page 19 of 26 ACS Applied Materials & Interfaces

1 2 3 Funding Sources 4 5 6 This work was financially supported by the National Natural Science Foundation of China (No. 7 8 51304166), the Australian Research Council Discovery Program, the Australian Research 9 10 Council Discovery Early Career Researcher Award scheme (DE150101617 and DE140100716), 11 12 13 and Deakin University Central Research Grants Scheme. 14 15 16 Notes 17 18 19 The authors declare no competing financial interests 20 21 22 ACKNOWLEDGMENT 23 24 We thank R. V. Driel for helping us to prepare the samples for HRTEM measurements. 25 26 27 REFERENCES 28 29 30 (1) Kuang, Z.; Chen, Y.; Lu, Y.; Liu, Y.; Hu, S.; Wen, S.; Mao. Y; Zhang. L.; Fabrication of 31 32 highly oriented hexagonal boron nitride nanosheet/elastomer nanocomposites with high thermal 33 34 conductivity. Small 2015, 11, 16551659. 35 36 37 (2) Renteria, J. D.; Ramirez, S.; Malekpour, H.; Alonso, B.; Centeno, A.; Zurutuza, A.; 38 39 Cocemasov, A. I.; Nika, D. L.; Balandin, A. A. Strongly anisotropic thermal conductivity of 40 41 42 freestanding reduced graphene oxide films annealed at high temperature. Adv. Funct. Mater. 43 44 2015, 25, 4664–4672. 45 46 (3) Lee, S.; Yang, F.; Suh, J.; Yang, S.; Lee, Y.; Li, G.; Choe, H. S.; Suslu, A.; Chen, Y.; Ko, 47 48 49 C.; Park, J.; Liu, K.; Li, J.; Hippalgaonkar, K.; Urban, J. J.; Tongay, S.; Wu, J. Anisotropic in 50 51 plane thermal conductivity of black phosphorus nanoribbons at temperatures higher than 100 K. 52 53 Nat. Commun. 2015, 6, 8573. 54 55 56 57 58 59 60 ACS Paragon Plus Environment 19 ACS Applied Materials & Interfaces Page 20 of 26

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